Turbochargers are used to increase the intake air pressure of internal combustion engines, and are increasingly being used to increase internal combustion engine output with lower engine displacements and improved fuel efficiency. A turbocharger includes a turbine wheel and a compressor wheel, generally mounted on a common shaft and disposed in separate housings. Engine exhaust is routed through the turbine where it drives a turbine wheel that generally includes an impeller having blades or vanes and is coupled, directly or indirectly, to a compressor wheel that also generally includes an impeller having blades or vanes. The compressor wheel draws in intake air, generally through a filtration system and into an inlet duct where it is drawn across the blades or vanes, compressed and supplied to the intake port or manifold of the engine. The compressor wheel spins at high rotational speeds, including speeds in the range of 100,000 to 150,000 revolutions per minute, or greater.
To increase compressor performance, bypass ports are added to the compressor inlet. These ports may be added in several forms, including as a ported shroud. A compressor without a bypass port generally has a single inlet to the compressor wheel that is defined by the compressor housing. A ported shroud bypass port provides a compressor inlet that has an inner and outer portion. A ported shroud bypass port compressor may have a housing similar to those of compressors that do not have a port, where the housing defines a compressor inlet and outlet, but it also has an additional outer wall separated from the (inner) inlet wall. In such configurations, the compressor wheel is mounted in a central portion of the compressor housing within the inner wall of the inlet and the bypass port is defined by an additional outer wall that forms a shroud around the inner wall of the compressor housing. The inner wall extends beyond the compressor wheel, but does not extend as far outwardly as the outer wall. The bypass portion of the inlet or bypass channel lies between the outer surface of the inner wall and the inner surface of the outer wall. The main or inner portion of the inlet includes a central channel, defined within the inner surface of the inner wall and provides a path to the face of the compressor wheel. The inner portion of the inlet also has a channel, or channels, defined between the main inlet and the inner surface of the inner wall, through the wall to the outer surface of the inner wall that fluidly connects the bypass portion of the inlet, and the bypass port. The annular channel(s) open into the inner surface of inner wall proximate the vanes or blades of the compressor wheel.
A bypass port increases the operating range of a compressor by expanding the extent of both its low mass flow range and the high mass flow range. The low mass flow range is limited by a phenomena referred to as “surge,” where the volume of air provided to the compressor exceeds the system requirements, and is limited at high mass flow by a phenomena referred to as “choke,” where the system's air requirements exceed the maximum flow rate of the compressor. The annular channel, or port, in communication with the compressor wheel acts as a bypass. At low mass flows, which would otherwise cause a surge condition without the bypass port, the presence of the bypass port allows flow back from the compressor wheel to the main inlet, thereby allowing the system to reach equilibrium at lowest mass flows. At high mass flows, which would otherwise cause a choke condition without the bypass port, the presence of the port allows extra air to be drawn directly into the bypass port from the main inlet and supplied to the blades of the compressor wheel. Due to the extended operational range, compressors configured with this type of inlet are sometimes known as “map width enhanced” compressors.
However, the use of a bypass port also increases the noise generated by the compressor, since the port provides a direct sound path to the compressor wheel, and thus provides a means for audible noise (sound waves) generated by the compressor wheel at high rotational speeds and mass flows or pressure ratios to exit the compressor housing. This high speed rotation of the turbine and compressor wheels causes the turbine and compressor blades to generate high levels of noise, known as Blade Pass Frequency noise, or sometimes informally referred to as turbo whine. One method of reducing this noise has been to place an annular inner deflector in the bypass port between the inner wall and outer wall that projects both orthogonally into the port and that extends axially along the port, thereby creating a “torturous” path for the air and sound waves to traverse. Another solution has been to add an annular noise suppressor ring to the inner surface of the outer wall that has an inner diameter that is less than the inner diameter of the bypass port, i.e., the outer diameter of the inner wall, in order to block line-of-sight transmissions of sound out of the annular channel comprising the bypass port.
While these features are effective to reduce noise associated with high speed rotation of the compressor under choke conditions, they were not designed, nor are they effective to, control gas flows within the bypass port particularly where these flows exit the bypass channel into the main inlet channel as occurs under surge conditions, i.e., low mass flow operation of the compressor.
Accordingly, it is desirable to control gas flow through the bypass port into the main compressor inlet and provide compressors and turbochargers having control features that provide such control.